The invention relates to vehicle steering systems.
Most modern motor vehicles are provided with entirely mechanical or semi-mechanical steering systems. Mechanical systems generally include a steering wheel fixed to a steering column, the rotation of which controls the movement of wheels of the vehicle via a steering linkage including a steering box which converts the rotary movement of the steering wheel to longitudinal movement of track rods which control the movement of the wheels. Since it would necessitate much effort by the driver to move the wheels if the steering wheel was directly connected to the steering linkage, a reduction gear system is generally incorporated in the steering box to assist the driver.
There are disadvantages and problems with known mechanical systems. For example, much effort by the driver is required to turn the wheels in certain conditions, even when a reduction gear system is included, particularly where the vehicle is stationary. The driver is also subject to shocks being transmitted via the steering column to the driver's hands when the wheels hit a pot hole or obstruction in the road, thus making driving difficult and possibly dangerous.
On heavy cars and larger vehicles power assisted steering is used, a hydraulic, electrical or air system being included to assist movement of the wheels when the effort required at the steering wheel would otherwise be excessive.
Power assisted steering systems help to remove some of the disadvantages mentioned above, but such systems tend to be very expensive and bulky and the combination is often heavy, requiring specifically designed parts. The driver is also at some risk if there is a failure in the system.
In aircraft systems have been developed for manipulating moveable parts such as ailerons and rudders on which the input from the pilot is decoupled from the moving part, and forces acting on the part are absorbed by the system. Because of the growing size of aircraft and the need for lightweight components, the mechanical systems previously used became unwieldy and unsuitable and were entirely replaced by hydraulic systems which were lighter and more versatile.
Because of the advantages provided by entirely hydraulic systems over mechanical systems such as their reliability, efficiency, light weight, and their usefulness in offering a more flexible and versatile means of power transmission, it has been sought to use such systems wherever possible. However, because of the possibility of failure resulting from a drop in pressure in the system due for example to leakage or contamination of the hydraulic fluid, the sole use of such systems has been avoided in mobile situations where total control of the moving parts is vital to safety, e.g. the steering of a motor vehicle.
In EP-A-0225773 there is described a vehicle steering system for hydraulically steered rear wheels, which in the event of failure operates to steer the rear wheels straight ahead. However, the vehicle also comprises conventionally steerable front wheels under direct control by the driver.
Furthermore, it has been noted that different drivers have different preferences with regard to the characteristics of steering systems, and hitherto it has not been possible to adapt the characteristics of a system to suit the driver. When a driver feels comfortable his driving will probably tend to be safer.